Semiconductor optical components have been extensively developed for use as optical emitters and detectors, particularly for discrete devices. The use of III-V semiconductor compounds allows the wavelength of these devices to be tailored to particular needs. There is strong desire in the field, however, to integrate a number of optical components on a single semiconductor device, that is, a desire for monolithic integration of both active and passive optical devices in a single integrated optical circuit. Such integration presents a significant difficulty for optical components. Optical emitters, e.g., diode lasers, and optical detectors, e.g., photosensitive diodes, are active in a wavelength region in which the active layer of the component is strongly light absorbing. For diode lasers, the lasing wavelength is closely related to the bandgap E.sub.g of the active layer. Similarly, photosensitive diodes respond primarily to light of higher energy than the bandgap of their active layers. In contrast, passive optical components, e.g., optical waveguides, require a waveguiding layer which is non-absorbing. In terms of bandgap, the waveguiding layer must have a bandgap larger than the energy of the light it transmits.
These differing requirements of the bandgaps of the active and transmitting (transparent) layers have necessitated relatively complex optical integrated circuits and long processes for fabricating them. For the most part, the requirement is met by depositing one III-V compound for the laser active region and another III-V compound for the waveguiding layer. There then arises the problem of coupling the light from the active laser region to the waveguide region and the difficulty of multi-growth steps.
One example of coupled optical devices is a stripe laser array. A typical semiconductor laser extends as an active layer between two current injecting layers. In order to obtain a high power beam with a stable output mode, it is well known to form a multiplicity of parallel stripe lasers. However, phase jitter between the lasers can be a problem and in some structures it is typical that a stripe laser is 180.degree. out of phase with another closely spaced stripe laser, resulting in a double-lobed far field output pattern. This problem has been at least partially solved by diffraction coupling the optical outputs of the strips lasers. For instance, Katz et al discloses in a technical article entitled "Diffraction coupled phase-locked semiconductor laser array" appearing in Applied Physics Letters, volume 42, 1983 at pages 554-556 a laser structure which is generally planar so as to confine light vertically and to vertically inject current. However, the upper cladding layer is etched over only a central area so as to form parallel horizontal waveguides. The waveguides control the modes but the cross-coupling in the horizontally non-defined end sections locks the phase between the stripes. Nonetheless, the Katz approach injects current into the end sections, which otherwise strongly absorbs light emitter by the array section.